Bacteria could survive impact shockwave

[Brian G Turner]

Another finding in support of panspermia:
Microbes might thrive after crash-landing on board a meteorite

However, if an asteroid or comet is breaking up in a planetary atmosphere, microbes resident on it could end up easily spread about long before any impact event.

 

[Vertigo]

I still have a huge problem with the panspermia theories as all they do is push the problem back; even if life did arrive here from space it must have originated somewhere else first. Then it not only has to survive whatever event got it into space first (see below) but then has to survive in space for the time it takes to cross interstellar space over probably millions of years rather than thousands, with exposure to massive radiation in the process, and only then survive the landing on Earth. Frankly surviving that landing strikes me as the least of its problems.

No one has ever come up with a mechanism for life to have evolved in space (at least not our carbon/hydrogen/oxygen life) whereas there are several viable theories of how it could have evolved on a rocky planet with abundant water. I'm afraid my Occam's Razor suggests that is far more likely than panspermia.

 

[Dennis E. Taylor]

There is weak panspermia and strong panspermia (kind of like the anthropic principle). Strong panspermia posits actual life coming to Earth, and I agree that's just a layer of turtles.
Weak panspermia, OTOH, doesn't posit life as such, but pre-life biological chemical units. And some fairly complicated chemistry has been detected in space, both spectroscopically and by analysis of comets. So the idea is that complex chemistry develops in space, where there's a lot of raw materials, energy, and time, and rains down on protoplanets. Most of the early stuff gets toasted, but once the planet cools enough, this prebiological soup becomes the basis for the development of life.
If you assume (not a given, but a reasonable assumption) that DNA and other aspects of our biology are inevitable, given the way chemistry works, then it is quite possible that life on widely separated planets could resemble each other, at least at the subcellular level. It doesn't mean we could interbreed with Romulans, but it does mean we might be able to eat them. And vice-versa, unfortunately.

 

[Vertigo]

I have no problem with that weak definition but then I still think such complex chemistry would form much more readily around alkaline thermal vents than in space and that actually there is little need to have any contribution from space. Which, of course, is not the same thing as saying it didn't happen

 

[Brian G Turner]

all they do is push the problem back; even if life did arrive here from space it must have originated somewhere else first.

Weak panspermia, OTOH, doesn't posit life as such, but pre-life biological chemical units.

Indeed, IMO it would be unscientific to presume a single source of all life. Instead, life must form where conditions allow as a natural consequence of physical laws.

However, not all niches might necessarily be populated due to various barriers, and this is where panspermia comes in as a means of transmission.

Additionally, all life may not be created equally - the injection of different genetic material into an existing system might have a profound effect on how that system develops and evolves.

 

[Flannery]

I still have a huge problem with the panspermia theories as all they do is push the problem back; even if life did arrive here from space it must have originated somewhere else first. Then it not only has to survive whatever event got it into space first (see below) but then has to survive in space for the time it takes to cross interstellar space over probably millions of years rather than thousands, with exposure to massive radiation in the process, and only then survive the landing on Earth. Frankly surviving that landing strikes me as the least of its problems.

Frankly, this paragraph alone, as a writer, is giving me all sorts of plot bunnies... Off to make notes!

 

[Vertigo]

Indeed, IMO it would be unscientific to presume a single source of all life. Instead, life must form where conditions allow as a natural consequence of physical laws.

However, not all niches might necessarily be populated due to various barriers, and this is where panspermia comes in as a means of transmission.

Additionally, all life may not be created equally - the injection of different genetic material into an existing system might have a profound effect on how that system develops and evolves.

I would agree with that accept the evidence shows that all life on earth from prokaryotes (bacteria and archaea) to eukayotes (all complex life from fungi to plants to animals) share the same genes; they are all related. Now prokaryotes (bacteria and archaea) happily pass their genes between each other; the genes literally pass through the cell wall and join the existing genome mix in that cell. So it is conceivable that panspermia genes could have merged with prokaryote genomes in the past using this mechanism. However eukaryotes do not support this mechanism, they can only inherit genes, so the only way new genes can appear is through mutation (the normal mechanism of evolution). Since eukaryotes evolved from a chance endosymbiosis between a bacterium and an archaea some 2 billion years ago and if panspermia had been significant over that time (roughly half the time that life has existed on Earth) we should expect to see significant genetic divergence in the prokaryotes compared to eukaryotes and we simply don't see that at all. All the indications are that, on Earth at least, all life is related, and that all complex life is derived from one single chance event; a chimeric merging of two of the three main domains of life bacteria and archaea.

So whilst there might be evidence that microbes can survive crossing space and can survive atmospheric entry there's little evidence that it actually did.

Frankly, this paragraph alone, as a writer, is giving me all sorts of plot bunnies... Off to make notes!

If you want life based plot bunnies I recommend you take a look at Nick Lane's book The Vital Question. Our own biochemical evolution reads almost like a science fiction adventure in its own right

 

[Brian G Turner]

we should expect to see significant genetic divergence in the prokaryotes compared to eukaryotes

I'm not sure what you mean - do you simply mean that the amino base in genetic code doesn't deviate from ACTG?

However eukaryotes do not support this mechanism, they can only inherit genes, so the only way new genes can appear is through mutation

Eukaryotes can assimilate new genes from their environment through horizontal gene transfer, rather than inherited through reproduction. It's only been discovered/researched over the past 15 years or so:
Horizontal gene transfer - Wikipedia

Annoyingly, it's one of those things I originally predicted in my (unpublished) writing 20 years ago, as a necessary aspect to panspermia. Luckily, I still have a few big predictions in the writing I'm publishing now.

 

[Vertigo]

You are quite correct it does happen in eukaryotes, I apologise, but much less so than prokaryotes and mostly (though certainly not exclusively) only in single celled organisms. What I was trying to say is that, after 2 billion odd years, we would expect see many radically different panspermia sourced genes in prokaryotes that don't appear in eukaryotes which, to the best of my knowledge, simply isn't the case. Also I'm sadly very sceptical about the idea of such complex organics as genes coming from space and proving to be compatible with our DNA.

However the bottom line is that we can never prove a negative so we can never say absolutely that panspermia has never happened and if ever some form of panspermia is proven that will certainly be a fascinating discovery.

 

[Brian G Turner]

When I think about genetic material coming from space I think more in terms of rogue RNA or viruses. IMO panspermia needs a more aggressive vector than bacteria and similar to be feasible. The more basic the source, the more likely any information it carries might be taken up into hosts. This would especially be essential for complex organisms.

2c.

 

[Brian G Turner]

Here's an interesting report from New Scientist about how plant RNA in pollen determines that a bee will become a worker:
Bee larvae fed beebread have no chance of becoming queen